1. Field of the Invention
The present invention relates to an image forming apparatus provided with a fixing unit for permitting a sheet bearing a toner image to pass between a heating member and a pressing member to heat and melt unfixed toner and fixe it to the sheet.
2. Description of the Related Art
In recent years, attention has been focused of belt-type image forming apparatuses, in which a smaller heat capacity can be set, due to demands of shortening a warm-up time and saving energy in a fixing unit (see, for example, Japanese Unexamined Patent Publication No. H06-318001). Attention has been also focused on an electromagnetic induction heating method (IH) with a possibility of quick heating and high efficiency heating in recent years, and many products as a combination of electromagnetic induction heating and the employment of a belt have commercialized in light of saving energy upon fixing a color image. In the case of combining the employment of a belt and electromagnetic induction heating, an electromagnetic induction device is often arranged at an outer side of the belt due to merits that a coil can be easily laid out and cooled and further the belt can be directly heated (so-called external IH).
In the above electromagnetic induction heating method, various technologies have been developed to prevent an excessive temperature increase in a sheet non-passage area in consideration of a sheet width (paper width) passed through the fixing unit. Particularly, the following prior arts are known as size switching means in the external IH.
A first prior art (Japanese Unexamined Patent Publication No. 2003-107941) discloses that a magnetic member is divided into a plurality of pieces, which are arranged in a sheet width direction, and some of the magnetic member pieces are moved toward or away from an exciting coil in accordance with the size of a sheet to be passed (paper width). In this case, heating efficiency decreases by moving the magnetic member pieces away from the exciting coil in sheet non-passage areas, and the amount of heat generation is thought to be less than in an area corresponding to a sheet with a minimum paper width.
A second prior art (Publication of Japanese Patent No. 3527442) discloses that other conductive members are arranged outside a minimum paper width in a heating roller and the positions thereof are switched between those inside and outside the extent of a magnetic field. According to the second prior art, the conductive members are first located outside the extent of the magnetic field to heat the heating roller by electromagnetic induction. If the temperature of the heating roller rises to the vicinity of a Curie temperature, the conductive members are moved to the extent of the magnetic field. Then, magnetic flux leaks from the heating roller at the outer sides of the minimum paper width, thereby preventing excessive temperature increases in the sheet non-passage areas.
However, the first prior art has a problem of inadvertently enlarging the entire apparatus since the movable range of the magnetic member is large and an extra space is, accordingly necessary. On the other hand, the second prior art can save space since the members for switching the size are arranged in the heating roller. However, the interior of the heating roller is a high-temperature environment and it is necessary to set a high Curie temperature in the case of arranging a certain member therein. Above all, a member with large heat capacity has a problem of extending a warm-up time.